As David said, you will not really have the time to do a fit. You need to gain access, without being seen, you need to get an ariel to a good place, you need to wire it up, and so on. You also need to make sure you leave no markings, no damage, and if the car is dirty, you buggered. I'll give a ballpark figure, 4 hours to complete a fit start to finnish. Don't question it, it's validity, it's accuracy, believe me I know.
You would be far more able to produce and fit a device with magnets and attach it to the bottom of the car. Don't forget we are still in illegal territory here though, If seen you might get picked up under suspision of planting an IED, and spend a day in the clink.
But on the engineering side your device could be movement activated and transmit short bursts of coded data to save on the power side. A quater wave ariel on one of these modules will be around 30cm long, can you see the difficulty in it's deployment now? It's not a good idea anyway!